Question

why do oxygen atoms share electrons in o₂ instead of transferring them like sodium and chlorine?

Explanation:

Oxygen atoms have 6 valence electrons. To achieve a stable octet (8 valence electrons), each oxygen atom needs 2 more electrons. If they tried to transfer electrons, each would need to either lose 6 (unlikely, as it takes much energy) or gain 2. But two oxygen atoms can share 2 pairs of electrons (a double covalent bond), each contributing 2 electrons to the shared pairs. This way, both attain an octet. In contrast, sodium (metal, low electronegativity) transfers its 1 valence electron to chlorine (non - metal, high electronegativity) because the electronegativity difference is large, forming ions held by ionic bonds. Oxygen atoms have similar electronegativity, so electron sharing (covalent bonding) is more favorable than transfer.

Answer:

Oxygen atoms share electrons in $\ce{O2}$ (form a covalent bond) because they have similar electronegativities, and sharing 2 pairs of electrons (a double bond) allows both to achieve a stable octet. Sodium (metal, low electronegativity) transfers its valence electron to chlorine (non - metal, high electronegativity) due to a large electronegativity difference, forming an ionic bond.